1. Technical Field
The present invention relates to a power source switching unit and a computer, and more particularly to a power source switching unit for switching power paths for a plurality of batteries to be charged with electric power supplied from the outside, and a computer equipped with that power source switching unit.
2. Description of the Related Art
In notebook-sized personal computers (hereinafter referred to as notebook-sized PCs), in order to enhance their portability, it is becoming standard to prepare an AC adapter (AC/DC converter), which converts commercial power to DC power to supply it to the computer main body, separately from the notebook-sized PC, and to install and employ the AC adapter in the notebook-sized PC as needed.
There are many cases where such a notebook-sized PC, because of its excellent portability, is employed where no commercial electric power is obtained. To cope with these cases, etc., some of the notebook-sized PCs are equipped with a plurality of batteries, such as a main battery, a second battery, etc., in which DC power obtained by the above-mentioned AC adapter is charged. When no AC adapter is installed, DC power is supplied by employing any one of the above-mentioned plurality of batteries.
In this kind of notebook-sized PC, in charging the above-mentioned plurality of batteries, rapid charging is generally performed until the batteries are fully charged, after trickle charging has been performed until battery voltage reaches a constant value. In trickle charging, a small amount of charging is performed so that a battery is not damaged, and during trickle charging, the capacity of a battery is near 0 (zero). Therefore, a battery that is executing trickle charging cannot supply the electric power required for operating the system.
FIG. 8 shows an example of a power-source switching unit for a notebook-sized PC, equipped with two batteries. As shown in the same figure, this power-source switching unit is equipped with a first series circuit 100 provided between a power-supply line L and a main battery 64A, and a second series circuit 102 provided between the power-supply line L and a second battery 64B. The power-supply line L leads from an AC adapter 62 to a DC-DC converter 66 in which an input DC voltage is converted to a predetermined voltage to be employed in each part of the notebook-sized PC.
The first series circuit 100 is equipped with a field effect transistor 1 (hereinafter referred to as a FET1) and a FET2. The second series circuit 102, as with the first series circuit 100, is equipped with a FET3 and a FET4.
The FET1 and the FET3 have body diodes D1 and D3 wherein the cathode is connected to a drain D and also the anode is connected to a source S. The FET2 and FET4 have body diodes D2 and D4 in which the cathode is connected to a source S and also the anode is connected to a drain D.
On the other hand, a trickle charging circuit 140A is provided between the power-supply line L and the source S of the FET1, and a trickle charging circuit 140B is provided between the power-supply line L and the source S of the FET3. A rapid charging circuit 142 is provided between the power-supply line L and the drain D of the FET2. Note that the drains D of the FET2 and the FET4 are connected with each other. Also, between this point of connection and the power-supply line L, a FET5 is provided for preventing the short circuit of a rapid charging circuit 142 which is performing rapid charging. That is, the FET5 is switched off when the main battery 64A or second battery 64B is rapidly charged by the rapid charging circuit 142, and is switched on, when the main battery 64A or second battery 64B is trickle charged by the trickle charging circuit 140A or 140B, or when DC power is supplied from either the main battery 64A or the second battery 64B to the DC-DC converter 66.
In the power-source switching unit constructed as described supra, in the case where the system is in operation, and the AC adapter 62, the main battery 64A in a full charged state, and the second battery 64B in an empty state have been installed, the trickle charging circuit 140B performs trickle charging on the second battery 64B. When this occurs, the FET1 and the EFT3 are both off, the FET2 and the FET4 are both on, and furthermore, the FET 5 is on.
Therefore, in the case where in this state the electric power supplied from the outside is intercepted by disconnection of the AC adapter 62 from the system, DC power is to be supplied from the main battery 64A, through the body diode D1 of the FET1, the FET2, and the FET 5 in sequence, and to the DC-DC converter 66.
However, in the case where the power-source switching unit is equipped with both the trickle charging circuit and the rapid charging circuit, as shown in FIG. 8, there is a problem that the power-source switching unit will be increased in cost and difficult to reduce in size.
To overcome this problem, it is considered that a trickle charging circuit and a rapid charging circuit are constructed and employed as a single charging circuit (hereinafter referred to as an “integrated charging circuit”). However, this case has the following functional problems.
FIG. 9 shows an example of a power-source switching unit equipped with an integrated charging circuit. This power-source switching unit differs from that shown in FIG. 8, in that the trickle charging circuit is omitted and that the rapid charging circuit is replaced with an integrated charging circuit 144. Note that the power-source switching unit shown in the same figure is constructed such that a power management controller (hereinafter referred to as a “PMC”) 104 controls the switched states of the FET1, the FET2, the FET3, the FET4, and the FET5 through the FET driving circuits.
FIG. 10 shows an example of the charging characteristic of the above-mentioned integrated charging circuit 144 in the case where batteries to be charged are constructed by connecting 3 (three) lithium ion batteries of rated voltage 4.2 V in series. As shown in the same figure, after trickle charging has been performed with a charging current value of 0.3 A until the charging voltage reaches 9.0 V (3.0 V per lithium ion battery), rapid charging is performed with a charging current value of 2.8 A until the charging voltage reaches a full charging voltage (12.6 V).
In the power-source switching unit constructed as described supra, in the case where the system is in operation, and the AC adapter 62, the main battery 64A in a full charged state, and the second battery 64B in an empty state have been installed, the AC adapter 62 supplies electric power to the DC-DC converter 66, and the integrated charging circuit 144 performs trickle charging on the second battery 64B by the electric power supplied from the AC adapter 62. When this occurs, the FET3 and the EFT4 are both on in order to charge the second battery 64B, and the FET1 and the FET2 are both off in order to prevent a short circuit between the main battery 64A and the second battery 64B. In addition, the FET5 is off in order to prevent the short circuit of the integrated charging circuit 144.
In the case where in this state the electric power supplied from the outside is intercepted by disconnection of the AC adapter 62 from the system, the FET1 and the FET2 are both off and therefore the main battery 64A is disconnected from the system. Because of this, the supply of DC power from the main battery 64A to the DC-DC converter 66 cannot be performed. Therefore, in this case, the system will be shut down.
The present invention has been made in order to overcome the above-mentioned problems. Accordingly, it is an object of the present invention to obtain a power-source switching unit which is capable of cost reduction and size reduction and also supplying electric power reliably even when external power supply is intercepted. Another object of the present invention is to obtain a computer that is capable of avoiding shutdown which results from the interception of external power supply.